The present invention relates to a process for the recovery of zinc, copper and cadmium from their ferrites by treating the ferrites under atmospheric conditions in a sulfuric acid bearing solution in the presence of potassium, sodium or ammonium ions at 80.degree.-105.degree. C. in order to precipitate, as jarosite, the iron present in the ferrites, by extracting at least a portion of the jarosite-bearing solid from the solution before the solution is returned to the neutral leach, to which acid and calcine are also fed and from which the solution containing zinc, copper and cadmium is recovered, and by feeding the solid obtained from the neutral leach to the said ferrite treatment stage.
U.S. Pat. No. 3,959,437 and U.S. patent application Ser. No. 771,661 disclose a leaching process for zinc calcine, which is divided into a neutral leach and a treatment stage for ferritic solid.
The process of U.S. Pat. No. 3,959,437 generally proceeds as follows: a raw material generally referred to as calcine, containing the ferrite of non-ferrous metal as well as the oxide of the non-ferrous metal is subjected to a neutral leach which substantially dissolves the oxide, but leaves the ferrite substantially unaffected. The non-ferrous metal values in solution are recovered and the undissolved ferrite material is further treated in a "conversion" stage with sulfuric acid bearing solution at atmospheric pressure and at a temperature of about 80.degree. C. to about 105.degree. C. in the presence of alkali or NH.sub.4 + ions. Under these conditions the non-ferrous metals dissolve as sulfates while iron is simultaneously precipitated as an insoluble complex sulfate known as jarosite which can be readily separated from the solution. The process has the advantages of simplicity in that it omits steps previously thought necessary in the removal of ferritic iron from calcine and of elegance in its one-step "conversion" of the ferritic material to jarosite.
The purpose of the neutral leach is to dissolve as completely as possible the principal component of the calcine, zinc oxide (ZnO), and to produce a raw solution, an iron-free zinc sulfate solution, having a pH value within the range 4-5.
During the neutral leach, not only the zinc oxide but also the zinc sulfate, which always constitutes a small percentange of the calcine, is dissolved on the other hand, the ferrites and unroasted sulfides of the calcine remain undissolved. The raw solution produced by the neutral leach contains, in addition to zinc, other heavy metals (Cu, Cd, Co, Ni, . . . ) which are regarded as impurities and which must be removed from the solution before the electrolysis in which the zinc is deposited on the cathode.
The ferritic solid remaining after the neutral leach of the calcine is fed according to U.S. Pat. No. 3,959,437 to its treatment stage to which a suitable quantity of electrolysis return acid, sulfuric acid and some suitable ammonium, sodium or potassium compound, usually their sulfates, are added. During the ferrite treatment stage the non-ferrous metals (Zn, Cu, Cd) of the ferrites pass into the solution as sulfates and the iron passes through the solution to the jarosite formed during the same stage. The treatment period and the reaction conditions are selected so that at the end of the stage the solid material is virtually devoid of ferrites and the iron content in the solution is so low that the solution can be returned directly to the neutral leach.
The following sum reaction occurs at the ferrite treatment stage EQU 3ZnO.Fe.sub.2 O.sub.3 (s)+6H.sub.2 SO.sub.4 (aq)+A.sub.2 SO.sub.4 (aq).fwdarw.2A[Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ](s)+3ZnSO.sub.4 (aq) (1)
(A=NH.sub.4, Na, K) PA1 T=temperature PA1 t=time PA1 []=concentration PA1 i=index expressing the particle size class PA1 S.sub.i =specific surface area of the particle size class PA1 f.sub.i =particle size class as proportion of total mass. PA1 x.sub.i =the extent of dissolving of particle size class i PA1 k=reaction velocity coefficient PA1 S.sub.io =specific surface area of particle size class at t=0. PA1 1. In general the ferrites of zinc calcine are finely-divided, in which case the bulk of the mass of each ferrite falls into particle size classes 1-4 (0-37 .mu.m), sometimes even into classes 1-3 (0-20 .mu.m). PA1 2. The specific surface areas S.sub.io of the same size classes of different ferrites deviate considerably from each other, i.e., the porosity and the particle shapes of the ferrites deviate considerably from each other. PA1 3. The dissolving of the coarser particle size classes of the ferrites (i=5-7 or 4-7), the proportion of the mass of which is usually within 0.2-0.4, clearly requires a longer time than would be necessary for the more finely-divided bulk of the ferrites. Thus, the complete dissolving of the coarser portion of the ferrites requires a long retention time, and in order to implement this, a relatively large reactor volume must be reserved even though the finely-divided bulk of the ferrite has already dissolved at the beginning of the reactor row.
and it consists of the partial reactions EQU 3ZnO.Fe.sub.2 O.sub.3 (s)+12H.sub.2 SO.sub.4 (aq).fwdarw.3ZnSO.sub.4 (aq)+3Fe.sub.2 (SO.sub.4).sub.3 (aq)+12H.sub.2 O(aq) (2)
and EQU 3Fe.sub.2 (SO.sub.4).sub.3 (aq)+A.sub.2 SO.sub.4 (aq)+12H.sub.2 O(aq).revreaction.2A[Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ](s)+6H.sub.2 SO.sub.4 (aq). (3)
In selecting the reaction conditions the aim is that Reactions (2) and (3) proceed during the same stage to as high a degree as possible, i.e. that the ferrites dissolve virtually completely and the iron which has thereby passed into the solution is precipitated almost completely as jarosite, in which case the sulfuric acid produced in the precipitation of iron is immediately consumed in the leaching of the ferrites. This procedure is disclosed in U.S. Pat. No. 3,959,437.
According to our studies the velocity of Reaction (2) depends on the ferrite concentration of the reaction mixture, on the particle size distribution of the ferrites, and on the sulfuric acid concentration and temperature of the solution. Generally the velocity r.sub.(2) of Reaction (2) can be expressed as follows: EQU r.sub.(2) =d[Fe].sub.(2) /dt=f.sub.(2) (T,[H.sub.2 SO.sub.4 ],[ZnFe.sub.2 O.sub.4 ],S.sub.i,f.sub.i), (4)
where
The dissolving velocity of each particle size class follows the formula EQU dx.sub.i /dt=k([H.sub.2 SO.sub.4 ],T).S.sub.io.(1-x.sub.i).sup.2/3( 5)
where
From Equation (5) we obtain EQU x.sub.i =1-(1-K.sub.i.t).sup.3 ( 6)
where EQU K.sub.i =1/3/S.sub.io.k([H.sub.2 SO.sub.4 ],T). (7)
By taking into account the particle size classes of the ferrites and the extents of dissolving, the following expression is obtained for the average extent of dissolving x of the entire ferritic material EQU x=.sub.i .SIGMA.f.sub.io.x.sub.i =.sub.i .SIGMA.f.sub.io.[1-(1-K.sub.i.t).sup.3 ] (8)
where f.sub.io is the proportion of the particle size class i of the total mass at t=0.
According to our studies the velocity of Reaction (3) for its part is dependent on the iron (III), sulfuric acid and A concentration of the solution, the jarosite concentration of the reaction mixture ([AJ]) and the temperature, i.e. it can be expressed as follows: EQU r.sub.(3) =-d[Fe].sub.(3) /dt=f.sub.(3) (T,[Fe],[H.sub.2 SO.sub.4 ],[AJ],[A]). (9)
According to our extensive experimental observations, the velocity r.sub.(3) of Reaction 3 at the ferrite treatment stage can be estimated relatively accurately using the function EQU r.sub.(3) =-d[Fe].sub.(3) /dt=k(T)[Fe].sup..alpha. [H.sub.2 SO.sub.4 ].sup..beta. [AJ].sup..gamma. ]A].sup..delta., (10)
where the exponents .alpha.,.beta.,.gamma. and .delta. are constant and the reaction velocity coefficient k is dependent on the temperature.
As shown in U.S. patent application Ser. No. 771,661, it was surprising that Reaction (3), i.e. the precipitation velocity of iron(III), is dependent on the jarosite concentration of the reaction mixture within a wide range of concentrations. In Equation (10) the exponent .gamma. of the concentration of A-jarosite (A=NH.sub.4, Na, K) has an approximate value of 1. Obviously Reaction (3) includes some stage which affects its velocity substantially and which occurs on the surface of the jarosite crystal, in which case an increased jarosite surface area increases the velocity of Reaction (3).
In U.S. patent application Ser. No. 771,661 it is shown how the dependence discussed above can be exploited in order to increase the velocity of Reaction (3) and thereby also that of Sum Reaction (1), the final aim being to achieve--by the procedure indicated in the claims of the patent application--an improved leaching process for zinc calcine.
Formulas (4)-(8) show that that part of the velocity of Reaction (2)--and thereby of Sum Reaction (1)--which is dependent on the particle size distribution of the ferrites--more precisely quantities f.sub.io and S.sub.io --cannot be affected by the process disclosed in U.S. patent application Ser. No. 771,661.
Some effect can be exerted on the particle size distribution of the ferrites by a selection of the roasting conditions within the limits of the roasting condition range available. Primarily the variables f.sub.io and S.sub.io seem, however, to be concentratespecific, i.e. determined by the composition and structure of the concentrate, which cannot be greatly affected by the selection of the roasting conditions, primarily the temperature--within the limits that are possible taking into consideration the roasting capacity.
Table 1 is a compilation of the particle size distributions of some ferritic solids. The ferrites were prepared by roasting, within the temperature range 900.degree.-1000.degree. C. and under oxidizing conditions, various zinc concentrates and by selectively leaching the oxides out of the calcines under conditions (T=80.degree. C., pH=1.5-2.0) in which ferrites remain undissolved. It will be seen from this table and from the examples that follow, that substantially all of the particles in the finer fraction are less than 37 .mu.m in size whereas most particles in the coarser fraction are larger than 37 .mu.m.
TABLE 1 ______________________________________ Particle size distributions of certain ferritic solids. i ##STR1## 1 2 ##STR2## 4 5 ______________________________________ 1 -5 0.570 0.419 0.187 0.038 0.211 2 5-10 0.035 0.056 0.215 0.217 0.185 3 10-20 0.035 0.063 0.083 0.070 0.238 4 20-37 0.063 0.088 0.208 0.313 0.246 5 37-74 0.114 0.109 0.147 0.129 0.082 6 74-149 0.153 0.148 0.128 0.170 0.031 7 149- 0.030 0.117 0.032 0.063 0.007 ______________________________________
Table 2 shows a compilation of the S.sub.io values of the ferrites of Table 1.
TABLE 2 ______________________________________ S.sub.io values of ferrites of Table 1 i ##STR3## 12345S.sub.io /m.sup.2 g.sup.-1 ______________________________________ 1 -5 7.7 5.7 0.58 2.0 0.46 2 5-10 3.5 2.5 0.44 1.1 0.30 3 10-20 2.7 1.6 0.38 0.76 0.24 4 20-37 1.9 0.88 0.33 0.55 0.19 5 37-74 1.3 0.70 0.28 0.40 0.15 6 74-149 0.91 0.37 0.24 0.28 0.12 7 149- 0.57 0.21 0.19 0.19 0.09 ______________________________________
We have explained the dependence of the reaction velocity constant k([H.sub.2 SO.sub.4 ], T) on the sulfuric acid concentration and the temperature, in which case, using Formula (8), the extent of dissolving x of the ferrites 1-5 and the extents of dissolving x.sub.i of the particle size classes can be calculated as funtions of time at certain values of the sulfuric acid concentration and temperature of the solution.
Table 3 shows quantities x and x.sub.i of the ferrites 1-5 as funtions of variable t. The sulfuric acid concentration and the temperature of the solution had values [H.sub.2 SO.sub.4 ]=40 g/l and T=95.degree. C. The column t.sub.xi=1.0 shows the time required for the complete dissolving of each particle size class.
TABLE 3 __________________________________________________________________________ x and x.sub.i values of ferrites 1-5 as functions of time where [H.sub.2 SO.sub.4 ] = 40 g/l and T = 95.degree. C. __________________________________________________________________________ ##STR4## f.sub.i (n) f.sub.i (k) ht.sub.xi=1.0 t = 0.5 h 1.0 h 2.0 h 4.0 hx.sub.i /-- 7.0 h 12.0 h 25.0 h __________________________________________________________________________ -5 0.570 0.811 0.66 0.985 1.0 5-10 0.035 0.050 1.5 0.711 0.966 1.0 10-20 0.035 0.050 1.9 0.600 0.894 1.0 20-37 0.063 0.089 2.7 0.458 0.749 0.982 1.0 37-74 0.114 3.8 0.341 0.594 0.889 1.0 74-149 0.153 5.6 0.245 0.446 0.734 0.977 1.0 +149 0.030 9.0 0.158 0.299 0.531 0.543 0.989 1.0 ##STR5## 0.717 0.827 0.931 0.983 0.999 1.0 1.0 ##STR6## 0.883 0.971 0.998 1.0 1.0 1.0 1.0 __________________________________________________________________________ ##STR7## f.sub.i (n) f.sub.i (k) ht.sub.xi=1.0 t = 0.5 h 1.0 h 2.0 h 4.0 hx.sub.i /-- 7.0 h 12.0 h 25.0 h __________________________________________________________________________ -5 0.419 0.669 0.90 0.913 1.0 1.0 5-10 0.056 0.089 2.0 0.574 0.871 1.0 10-20 0.063 0.101 3.2 0.403 0.680 0.950 1.0 20-37 0.088 0.141 5.8 0.237 0.434 0.719 0.970 1.0 37-74 0.109 7.4 0.190 0.355 0.614 0.905 0.999 1.0 74-149 0.148 14 0.104 0.201 0.373 0.640 0.879 0.998 1.0 +149 0.117 24 0.060 0.118 0.227 0.416 0.638 0.870 1.0 ##STR8## 0.504 0.631 0.747 0.865 0.940 0.984 1.0 ##STR9## 0.736 0.876 0.955 0.996 1.0 1.0 1.0 __________________________________________________________________________ ##STR10## f.sub.i (n) f.sub.i (k) ht.sub.xi=1.0 t = 0.5 h 1.0 h 2.0 h 4.0 hx.sub.i /-- 7.0 h 12.0 h 25.0 h __________________________________________________________________________ -5 0.178 0.270 8.8 0.161 0.304 0.539 0.838 0.992 1.0 5-10 0.215 0.310 12 0.124 0.237 0.433 0.718 0.937 1.0 10-20 0.083 0.120 14 0.107 0.205 0.381 0.650 0.887 0.998 1.0 20-37 0.208 0.300 16 0.093 0.179 0.336 0.587 0.831 0.987 1.0 37-74 0.147 18 0.081 0.157 0.296 0.527 0.769 0.961 1.0 74-149 0.128 21 0.069 0.135 0.258 0.467 0.701 0.919 1.0 +149 0.032 26 0.057 0.112 0.216 0.398 0.615 0.849 0.999 ##STR11## 0.107 0.206 0.379 0.637 0.856 0.976 .about.1.0 ##STR12## 0.123 0.234 0.426 0.703 0.914 0.996 1.0 __________________________________________________________________________ ##STR13## f.sub.i (n) f.sub.i (k) ht.sub.xi=1.0 t = 0.5 h 1.0 h 2.0 h 4.0 hx.sub.i /-- 7.0 h 12.0 h 25.0 h __________________________________________________________________________ -5 0.038 0.059 2.6 0.473 0.767 0.988 1.0 5-10 0.217 0.340 4.8 0.280 0.503 0.800 0.995 1.0 10-20 0.070 0.110 6.8 0.206 0.382 0.651 0.932 1.0 20-37 0.313 0.491 9.3 0.153 0.289 0.516 0.815 0.985 1.0 37-74 0.129 13 0.113 0.216 0.399 0.675 0.907 0.999 1.0 74-149 0.170 18 0.081 0.157 0.297 0.529 0.771 0.963 1.0 +149 0.063 28 0.053 0.105 0.202 0.375 0.587 0.820 0.999 ##STR14## 0.173 0.317 0.533 0.775 0.918 0.982 .about.1.0 ##STR15## 0.221 0.400 0.655 0.900 0.993 1.0 1.0 __________________________________________________________________________ ##STR16## f.sub.i (n) f.sub.i (k) ht.sub.xi=1.0 t = 0.5 h 1.0 h 2.0 h 4.0 hx.sub.i /-- 7.0 h 12.0 h 25.0 h __________________________________________________________________________ -5 0.211 0.333 11 0.129 0.247 0.450 0.739 0.950 1.0 5-10 0.185 0.292 17 0.085 0.166 0.312 0.552 0.795 0.974 1.0 10-20 0.238 0.375 22 0.068 0.132 0.253 0.459 0.691 0.912 1.0 20-37 0.246 27 0.054 0.107 0.206 0.381 0.592 0.827 0.999 37-74 0.082 34 0.044 0.086 0.167 0.314 0.501 0.731 0.982 74-149 0.031 43 0.034 0.068 0.133 0.254 0.413 0.625 0.926 +149 0.007 58 0.026 0.051 0.100 0.193 0.320 0.501 0.816 ##STR17## 0.077 0.150 0.282 0.496 0.714 0.895 0.995 ##STR18## 0.093 0.180 0.336 0.579 0.808 0.959 1.0 __________________________________________________________________________
Tables 1-3 show the results of our studies: